Any object having a temperature above 0 K will radiate infrared radiation to some extent. Depending on the temperature of the object, the infrared radiation will vary. If the temperature of the object is higher it will radiate more infrared radiation. Therefore, infrared sensors can remotely determine the temperature of an object by measuring the amount of emitted infrared radiation.
Integrated semiconductor infrared thermal sensors are known in the field which absorb the infrared radiation emitted by the object on a membrane which is isolated from the bulk silicon. This membrane will thus heat up due to the absorption of the received infrared radiation. The infrared radiation may furthermore be blocked from the surrounding bulk silicon by an aperture layer such that the bulk silicon only heats up negligibly relative to the membrane. The bulk silicon is furthermore typically substantially more voluminous than the membrane, such that even if the infrared radiation were not blocked, the temperature change in the bulk silicon would still be negligible under normal circumstances since the thermal resistance to the outside is so low that the bulk remains on ambient temperature. The bulk silicon can be seen as a heat sink on a constant temperature. Because the temperature of the bulk will be constant and equal to a reference temperature, e.g. the ambient temperature, the temperature difference between the bulk and the membrane increases when the membrane receives infrared radiation from an object. This temperature difference is therefore a measure for the temperature of the object. By measuring the temperature difference between the membrane and the bulk, the temperature of the object can be deducted.
The temperature difference between the membrane and the silicon bulk can be measured by placing thermocouples between the membrane and the bulk. A thermocouple will generate a voltage difference depending on the temperature difference between the two nodes of the thermocouple and the type of the material. Thermocouples may for example comprise n-type and p-type poly-silicon. It is also possible to place multiple thermocouples together in series to create a larger voltage difference.
Thermocouples may be placed on beams which connect the membrane to the bulk silicon in devices known in the art. All the thermocouples may be connected in a series circuit in order to obtain a large output voltage. However, the thermocouples also have a certain electrical resistance, e.g. depending on the doping, such that connecting all the thermocouples in series will also connect the resistances in series. The total electrical resistance will therefore be the sum of the electrical resistance of each thermocouple.
The temperature difference between the membrane and the silicon bulk is a measure to define the temperature of the object. It is thus advantageous to obtain a large temperature difference. Therefore, the thermal isolation between the membrane and the bulk is preferably very large.
The heat from the membrane can dissipate through the air above and under the membrane. Heat loss via this heat loss path can be reduced by increasing the etch depth, such that the conduction path will become larger and therefore a higher thermal isolation is achieved. Also, a lower pressure in the cavity around the membrane can be used, such that heat loss via conduction and convection is reduced.
The heat from the membrane also dissipates via the beams to the silicon bulk. Longer beams can be used to increase the thermal isolation. Furthermore, decreasing the width of the beams can also increase the isolation.
The conductivity of the beams is typically much higher than the conductivity of the air. However, the membrane has a much greater area exposed to air than to the beams, such that the thermal resistance of both the air and the beams can be important in infrared thermal sensor design.
As is known in the art, the thermal sensor cavity may be a high vacuum cavity. This offers the advantage of substantially reducing the heat loss via the membrane surface, e.g. conductive and convective heat loss are negligible, such that the membrane can only lose heat via radiative heat dissipation via the surface and through the beams. In such devices, it may be preferred to have narrow and long beams. However, for other devices, e.g. in which the cavity is filled with a low pressure gas, a good balance can be reached when the thermal resistance of the surface heat loss path is about equal to the thermal resistance of the beams.
This can be seen by considering the case where the thermal conduction of the air is dominant, such that substantially all the heat is dissipated through the air. In this case, thermocouples can be added to the design without changing the temperature difference between the membrane and the bulk. Since all the heat is conducting through air, adding or removing thermocouples will not change the temperature on the membrane. However, when the thermal conduction through the air is not dominant anymore, the temperature on the membrane will change when a thermocouple is added since the thermal resistance from the membrane to the bulk is changed. However, adding thermocouples will also increase the electrical resistance and therefore also the noise.
When starting from the other extreme in which the thermal conduction of the beams is dominant, e.g. such that substantially all the heat is lost through the beams, thermocouples should be removed from the design to increase the output voltage. Since all the heat is conducting through the beams, the thermal resistance between the membrane and the bulk increases when removing beams. When thermocouples are removed, the width of the remaining thermocouples can be decreased to keep the same total electrical resistance. However, when the thermal conduction is not dominated by the beams anymore, removing beams will not increase the thermal resistance as fast, because some heat will be lost through the air. Thus, an optimum could be reached when the thermal resistance of the air is about equal to the thermal resistance of the beams.
In order to obtain good thermal isolation of the membrane, the beams are preferably under-etched. For this, an anisotropic etchant such as tetramethyl ammonium hydroxide (TMAH) can be used. However, such anisotropic etchant may not etch well in a direction for which the material being etched has a dense configuration in the crystal lattice planes normal to that direction, for example, TMAH cannot etch silicon perpendicularly to the <111> direction. When a side of a window is opened along a <110> edge of a silicon surface cut in the <100> direction, the etched side will be formed by the <111> plane that intersects with the <110> edge. Furthermore, an anisotropic etchant such as TMAH may also not etch in an obtuse angle, although it can etch away a sharp angle very well.
It may be known in the art to use a plurality of holes, e.g. circular holes, provided in the membrane to allow passage of the anisotropic etchant. For example, the United States patent application US 2010/0289108 discloses a thermal sensor device comprising a membrane having such arrangement of etchant holes.
However, it may be a disadvantage of similar devices that a large portion of the surface exposed to incident radiation is removed by the etchant holes, and thus the heat storage capacity and the sensitivity of the device may be reduced. Also, such configuration of etchant holes may reduce the structural integrity of the membrane, e.g. the membrane may be prone to tearing under external forces, e.g. under torque.